A fast let down or oversupply would be my first thought too as a bottle is slower so it's probably staying down. Two of mine were lactose intolerant they couldn't tolerate breast feeding and my ds especially was failing to thrive with extremely low weight gains and lost more often than gaining any he was going to end up in hospital before we swapped to lactose free formula. We did this with a nutritionist/chiro and a pead. It's very uncommon to be lactose intolerant as it's been pointed out it's in breast milk. We think with ds it was because he was born with a low grade fever and infection that his tummy couldn't cope it also caused reflux you could smell the acid on his breath and dummy. He has outgrown it all now that he is older. Work on the oversupply theory first especially if your ds is gaining weight.